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    <!-- 求出1-1/2+1/3-1/4…..1/100的和 var i=1; -->
    <script>
        var num = 0;
        for(var i = 1;i <= 100; i++){
            if( i%2 == 0){
                num+= -1/i;
            }else{
                num+= 1/i;
            }
        }
        console.log(num);
        // 输出的结果居然能将所有的num的值相加，回去值得研究一下语法规定。
    </script>
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